DESCRIBE HOW WATER IS DRAWN FROM THE SOIL UP TO THE LEAVES. [20 M]

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  • Water is drawn into the root hair cells by osmosis, 
  • Due to the presence of dissolved substances in the cell sap of root hairs, the concentration of cell sap is greater than that of the surrounding solution in the soil / concentration gradient; 
  • This exerts a high osmotic pressure; thus drawing the water molecules across the cell wall and cell membrane into the root hair cells;
  • More water drawn into the root hair cells dilutes the cell sap making it less concentrated than that in the adjacent cortex of the root; 
  • Due to osmotic gradient water moves from the adjacent cells to the next by osmosis; until it enters into xylem vessels located in the centre of the root; 
  • These xylem vessels of the root then conduct the water up into the xylem vessels of the stem into the leaves.
  • There is a force in the roots called root pressure which pushes water up the stem; Energy is essential in this process; in the xylem vessels, water would rise by capillarity because the vessels are narrower and there is a higher attractive force between the water molecules and the cell walls; 
  • The cohesive and adhesive forces; are important in the maintenance of continuous and an uninterrupted water column in the xylem vessels up the tree to the leaves;
  • Water vapourises from the spongy mesophyll cells their cell sap becomes concentrated than adjacent cells; This increases the osmotic pressure of the spongy mesophyll cells; as a result water flows into the cell from other surrounding cells; which in turn take in water from xylem vessels in the item and roots; 
  • The transpiration pull maintains continuous column of water from the roots to the leaves. (maximum 20 Marks)
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